Patent Document 1 (Japanese Patent Publication (Kokoku) S61-33848) describes a method for producing an aqueous dispersion containing vinylidene fluoride [VdF]/TFE/hexafluoropropylene-based fluorine-containing elastomer particles with a particle diameter of 0.02 μm as dispersed therein which comprises carrying out the polymerization reaction in the presence of a fluorine-based surfactant having a polymerizable double bond represented by the formula CF2═CFO(CF2)nCOOM (in which n is an integer of 1 to 7 and M is an amine or alkali metal). However, there is no description about an aqueous dispersion containing a polymer having a high TFE monomer unit content.
Patent Document 2 (Japanese Kokai Publication H08-67795) describes a method for producing an aqueous dispersion containing VdF-based polymer particles with a particle diameter of not greater than 200 nm as dispersed therein at a concentration of not lower than 30% by mass which method comprises carrying out the polymerization reaction in the presence of at least one of various fluorine-containing surfactants having polymerizable double bonds. However, there is no description about an aqueous dispersion containing a polymer having a high TFE monomer unit content or about the fluorine-containing surfactant content in the aqueous VdF-based polymer dispersion obtained.
Patent Document 3 (Japanese Kokai Publication S60-250009), Patent Document 4 (Japanese Kokai Publication S62-288614) and Patent Document 5 (Japanese Kokai Publication S62-288616) respectively describe copolymers derived from a monomer represented by CF2═CFO(CF3)SO2F and TFE, copolymers derived from a monomer represented by CF2═CFOCF2CF(CF3)OCF2CF2SO2F and TFE, and copolymer derived from a monomer represented by CF2═CFOCF2(CF3)OCF2CF2SO2F and TFE. In each case, the polymerization is carried out in the presence of a relatively large amount of a fluorine-based surfactant and, further, there is no description referring to the fact that a stable aqueous dispersion containing a particle comprising a TFE polymer very small in diameter is obtained.